AC and DC electrical motors convert electrical energy into mechanical energy by utilizing the electromagnetic force produced by current carrying conductors within an electromagnetic circuit. This mechanical energy is typically in the form of a shaft undergoing rotation due to a changing electromagnetic field within the motor. The electromagnetic field change is caused by current variations in the electromagnetic circuit which is a function of the angular position of the shaft. The electromagnetic field incrementally changes its magnitude around the shaft inducing continuous rotation of the shaft.
To accomplish a change in current within the electromagnetic circuit, many electrical motors use a series of brushes to make electrical contact between components of the electromagnetic circuit located on the rotating shaft and other components which are stationary. The rotating commutator slides across the brushes providing an electrical connection between the rotating and stationary electronic components.
Generally, the brushes are made of carbon or a metallic material. Throughout the life of the motor, the repeated contact of the brushes causes wearing of the brushes and a release of brush particulates into the encapsulated environment within the motor housing. Build up of these electrically conductive particulates on the components of the electromagnetic circuit can cause an electrical short. Once an electrical short occurs, the efficiency of the motor degrades until the point in which it must undergo maintenance to clean and replace components within the motor. Additionally, the buildup can cause "arcing" of current within the motor which can be dangerous if the motor is used in a potentially explosive environment, such as a coal mine. And, large releases of current within the motor also present a safety hazard to individuals near the motor.
Fans are often placed near these motors to push air through openings in the motor housing to cool the motor and force the particulates from the motor housing. However, fans by themselves cannot solve the particulate problem because of an additional compound, lubrication, found in the encapsulated environment within the motor. Because the shafts rotate at thousands of revolutions per minute, various types of bearing assemblies containing lubrication are used to mount the shafts. Lubrication is exposed to the shaft and the air around the shaft since the bearing elements contact the shaft. The solid lubrication migrates along the shaft from the bearing assembly and is thrown from the rotating shaft. When the lubricant is released into the air, it settles on the electronic circuitry as the particulates do. The air movement by the fans cannot keep the particulates from "sticking" to the circuitry due to the mixture of lubrication and brush particulates. Not only can the particulate and lubricant mixture cause an electrical short due to the conductivity of the particulates, but the nonconductive lubricant can inhibit the electrical connection between the brushes and the rotating commutator.
In addition to the solid lubricant released into the environment surrounding the electronic circuitry, the drastic temperature rise during operation causes an increased outgassing of lubricant vapor from the solid lubricant. The liberated lubricant vapor then condenses back onto the colder surfaces within the housing. This process results in deposits of lubrication and brush particulates on all surfaces within the housing including the electronic circuit components.
Another problem with the motor occurs during bearing failure when the shaft cuts into the bearing cap and enlarges the through-hole through which the shaft passes. Thus, the minimal tolerance, usually about 1/32", between the outer periphery of the shaft and the through-hole can be greatly expanded to over 3/8". The expanded gap causes increased leakage of the lubricant from the bearings which results in a quicker failure of the bearings. Additionally, the particulates from the brushes can easily migrate into the bearing assembly through the expanded gap and lessen the effectiveness of the bearing assembly.
Considering that many of the motors are very large and weigh several thousand pounds, it can take several hours to access the surfaces coated with the lubricant and particulate mixture, clean those surfaces, and reassemble the motor. A vapor degreasing process is typically employed using various solvents such as trichloroethylene which releases hazardous vapors into the air. If the problem is not regularly addressed through periodic maintenance which is generally required every one to three months depending on the motor, then current arcing between the components damages the electromagnetic circuit. If the motor must be cleaned and the damaged components replaced, the motor "downtime" can last several days. Any "downtime" in the motor causes a downstream stoppage in the processes which rely upon the mechanical energy produced by the motor. In addition to the financial loss due to "downtime", the replacement of components and the man-hours necessary to perform repairs and maintenance is very costly.
As stated, the mere movement of air through the housing does not entirely alleviate the particulate build-up due to the presence of the lubricant. Throughout the motor industry, no attempt to isolate the lubricated bearings from the housing cavity has been successful. To accomplish this long felt need to eliminate the problems associated with the lubricant-particulate mixture and increase efficiency, many manufacturers have developed efficient "brushless" motors which utilize a series of electronic switches that open and close based on the angular position of the shaft which changes the current within the electromagnetic circuitry. Thus, with the removal of the brushes, no particulates are released. However, motors employing brushes continue to be produced. And, due to the high cost of new motors, the hundreds of thousands of existing brush electric motors continue to be used in every industry.
Additionally, attempts to retrofit existing motors by affixing a seal to the rotating shaft have been made, but have failed. The extreme vibration of the shaft loosens the seal and the means by which the seal is fastened to the shaft which ultimately leads to leakage of the lubricant into the housing cavity. In another retrofit attempt, a plastic seal fastened to a plate mounted on an inboard wall of the bearing assembly was utilized. Again, the vibration proved too much for the plastic seal.